Why are y'all saying Vegeta didn't know Gohan had become weaker until the tourney? He had already said it way before the two tournament statements:
Chapter: 423 (DBZ 229), P4.1
Vegeta: “Your body’s gotten rusty. No matter how peaceful it may be, you should still train just in case.”
As far as physicals go,
muscle mass is clearly not what Vegeta meant here. He's as buff as ever. His punches just don't pack the weight they would back in the Cell Games. Ie, his ki lowered.
He knew Gohan's strength had lowered ever since the Cell Games, and his statement regarding his ability to win even if nobody goes Super Saiyan was
precisely based on Gohan's power lowering.